So technically, yes, I have to believe in the existence of UFOs, as anything that flies and you don't know what it is counts as a UFO. Identifying such a thing as being from another planet is the hard part.

As for extra-terrestrials, I do believe, and a mountain of recent scientific research is supporting that belief. It is far more likely than not that planets that can support life exists in other solar systems. Once they crunch the probabilities of life-bearing planets in our galaxy along with the sheer number of stars in our galaxy, the possibility of life on other planets approaches near-certainty.

The real question should be: Since life on other planets is virtually guaranteed to exist, has any of them bothered to visit us yet? And my answer is a firm "Yes" given the massive amounts of stories and legends regarding alien visitors in some form or another throughout all recorded history of the human race.

Which in turn begs the question of how they got here, since we have a hard time even getting to another planet in our own solar system, much less visiting other solar systems.

Along with Cignus I have to agree that UFO's specifically are believable especially with my very limited knowledge of flying things.

Aliens, yes but that is different from extraterrestrials.
An alien is anything not from this planet which I believe do exist.
An extraterritorial is something from a different planet which I don't believe exist. Oddly enough the reason for this is the same reason I don't believe in time travel, I do believe in the constancy of time and I don't believe there is an absolute limit to the speed of light.